The cable core of this invention is for use in 20 hollow core cables through which oil or other fluid can be circulated for cooling the cable or for other purposes. The core is made of at least two layers of strips, preferably metal, which extend along helical courses with the angle of lay calculated so as to obtain a desired correlation between changes in the length of the metal strips and changes in the diameter of the core.
Because of the fact that plastic, such as is generally used for insulation or for jacketing around electrical cables, changes in dimension much more than metal such as is commonly used for shielding electric cables, provision has to be made to accommodate these differences in thermal expansion and contraction. If a metallic cable core or shield changes in diameter in direct proportion to the change in length of metal with a given temperature change, and the temperature change is substantial, then a plastic layer or jacket over the metal will be prevented from contracting to its full extent; and thermal stresses will be set up in the plastic which cause the plastic to crack when the stresses reach a critical value.
This invention provides a construction in which the diameter of the core contracts at a greater rate than the metal changes in length as the temperature of the metal changes. The change in core diameter can be made to more nearly approach the change in plastic jacket diameter for corresponding changes in temperature. Thus a hollow core cable made in accordance with this invention can be operated through a much wider temperature range without setting up destructive thermal stresses in a plastic jacket which is placed over the metal core.
For simplicity and economy, the cable core of this invention is made in such a way that the two layers of metal strips, which comprise the core, are related to one another in such a way as to provide a stable structure. This is done by providing the strips of each layer with a different direction of the helix. One helix is a right hand helix and the other is a left hand helix. If they unwind, therefore, they must rotate in opposite directions. The strips are then connected to one another at the ends of the core so that any tendency of either helix to unwind is offset by the other helix.
In addition to the right hand and left hand helixes, further stability of structure is obtained by having a different angle of lay for each helix so that their diameters do not change by equal amounts for a given change in temperature. In the preferred construction, the diameter of the outer layer changes less than that of the inner layer for a given temperature change; and thus the outer layer acts as a restraint on the inner layer when the temperature of the core increases.
With decrease in temperature, the inner layer contracts more than the outer layer and its pressure against the outer layer decreases with resulting decrease in friction between the layers at lower temperatures. The differences in diameter changes of one layer with respect to the other is kept at a low value so that the helical layers of strips of the two layers support each other back-to-back at all times.
The characteristics of the strips that make up the two layers of the core provide, within desired limits, provision for freedom of expansion and contraction and restriction of movement in opposite directions beyond the original state obtained by the judicious selection of characteristics of the interrelated helixes.
Another feature of the invention is the method of making the hollow cable core of this invention by winding the layers of strips of the hollow core on a "disappearing mandrel" which permits the making of cores of a thousand to two thousand feet in length which can be wound on a reel and from which core a tube mandrel could not be pulled out by any conventional means.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.